The hair-like projections on the outside of cells are called cilia. A single projection is a cilium, while cilia is the plural form. They are membrane-covered structures found on many eukaryotic cells.
Cilia have two broad functional classes. Motile cilia beat in coordinated waves to move fluid or particles across a cell surface. In the human respiratory tract, roughly 200 motile cilia may project from each epithelial cell, helping sweep mucus and trapped debris toward the throat. Similar cilia help move cerebrospinal fluid and assist the movement of an egg through the oviduct.
Primary cilia are usually non-motile and act more like microscopic sensory antennas. They detect chemical, mechanical and other signals outside the cell. Most primary cilia have a 9+0 arrangement of microtubules, while many motile cilia have a 9+2 arrangement.
Cilia are sometimes confused with flagella or microvilli. Flagella are generally longer and fewer in number, whereas microvilli are shorter, non-motile membrane projections supported by actin rather than the microtubule-based axoneme characteristic of cilia.